This invention relates to the cold forming, rolling, or reducing art and it has particular relationship to pilgering for reducing the cross sectional dimensions of tubes.
Fuel elements for nuclear reactors, particularly of the pressurized-water type (PWR) include a plurality of pellets of a compound of uranium or other fissionable material in cladding of a Zircaloy alloy, for example, Zircaloy-4 alloy. The cladding is a tube in which the pellets are deposited and which is seal welded at each end. In the fabrication of the cladding tube, a hollow cylinder of the alloy is originally produced by hot extrusion. The hot extruded tube is reduced in cross sectional dimensions (diameter and wall thickness) by multiple pilger reductions at essentially room temperature. Typically the tube has an outer diameter of 0.7 inch and a wall thickness of 0.070 inch before the last pilger reduction. It is necessary that the OD and the wall thickness be substantially reduced, typically to 0.375 inch and 0.023 inch, respectively, in the last pilger reduction. In reducing tubes of other materials than zirconium alloy for use as cladding, the initial reduction may take place by pilgering and the final reduction can be effected by cold drawing. Because of the texture requirements imposed on Zircaloy-4 alloy and other zirconium alloys used for cladding the final drawing operation is not feasible. The reduction of the hot extruded tube must be effected entirely by multiple pilger reductions to final size. Typically five pilgering operations are required to effect the typical reduction described above from the hot extruded tube to final tube size with intermediate annealing and chemical cleaning treatments.
Pilgering apparatus or pilger mills are described in Potapov et al. U.S. Pat. Nos. 4,184,352 and Matinlassi, 4,233,834. The pilgering apparatus includes a tapered mandrel on which the tube to be pilgered is mounted. There are also typically annular or ring dies. Each die has a peripheral circular groove of circularly arcuate, typically semi-circular, cross section. The transverse radius of the groove decreases progressively from a predetermined first point on the circumference to a predetermined second point which typically is substantially less than 360.degree. from the first point. The dies are mounted cooperatively rotatable on a roll stand or yoke. The yoke is oscillated forwardly and backwardly and the dies rotate in synchronism with the oscillations. As the dies rotate, their treads on the periphery are brought into tangency progressively around the periphery of the groove. The dies are so positioned on the roll stand that at each point of tangency the radius of the transverse cross section of the groove on one die is substantially equal to the corresponding radius of the other die. As the dies are rotated their grooves define a circular channel of progressively increasing or decreasing transverse cross section depending on the direction of rotation. The tapered mandrel with the tube mounted on it is inserted in this channel. The tube is advanced a short distance over the mandrel in steps following each full cycle of oscillation of the yoke. At the same time both the tube and the mandrel are rotated about their common axis typically by about 50.degree.. The pilgering process is similar, in its coaction between the tube to be reduced, the mandrel contained therein, and the die grooves to conventional sheet metal rolling.
The dies apply high pressure to the tube during the reduction. During the final, usually the fifth, pilger operation the pressure is typically about 200,000 pounds per square inch. To withstand this pressure the dies are composed of high strength tool steel, typically Bofors SR1855 which has the following nominal composition in weight percent:
Carbon--1.00 PA1 Manganese--0.80 PA1 Silicon--1.5 PA1 Chromium--1.00 PA1 Iron--Balance
Typically the mandrel is composed of another high-strength steel such as AISI-A8.
In the reduction of tubes of zirconium alloys by pilgering failure of the dies has been experienced. The failure was manifested first in spalling of the dies particularly before the dies were composed of the high strength SR1855 steel. Later the higher strength dies failed by developing cracks.
It is an object of this invention to overcome the above-described disadvantages and to provide pilgering apparatus and dies for such apparatus in whose use spalling and cracking of the dies shall be precluded or minimized.